In recent years, iron-copper-based oil-impregnated sintered bearings have been used also on output shafts of electric motors for automobiles. As such iron-copper-based oil-impregnated sintered bearing, those disclosed in, for example, Patent documents 1 and 3 are known. In the case of an automobile such as a minivan, power windows and wiper motors involving larger window glasses are operated with outputs higher than before, thus resulting in a higher load and surface pressure applied to a bearing of an output shaft. Further, there has been a problem that since the output shafts used in these motors are revolved at a low speed of 200 rpm or lower, an oil film unique to an oil-impregnated sintered bearing tends to be formed in an insufficient manner such that the wear resistance of the oil-impregnated sintered bearing is now at risk as the shaft and bearing slide against each other in the form of metal contact. In order to impart a lubricity to the iron-copper-based oil-impregnated sintered bearing under such sliding condition with a lesser oil lubrication effect, there has been known a technique of dispersing and distributing graphite in a bearing material. An iron-copper-based sintered bearing containing graphite is produced by press-molding a raw material powder(s) that has been mixed with a graphite powder; and then performing steps such as sintering, sizing and oil impregnation on a molded body obtained. However, since the material composition of the bearing is that of an iron-copper-based sintered alloy, an iron powder and a graphite powder react with each other during sintering such that a hard iron alloy phase can be formed easily. An iron-copper-based oil-impregnated sintered bearing having a hard alloy phase therein often damages a sliding partner shaft member and then undergoes wear itself due to the damaged shaft, under a sliding condition such as that involving an output shaft of an electric motor for an automobile. As a remedial measure, although a quenched steel member with a high hardness may be used as a shaft, a carbon steel shaft has been used for cost reduction.
Here, as a conventional technique for inhibiting a reaction between an iron powder and a graphite during sintering, there has been known a technique where a raw material powder is at first prepared by mixing an iron powder, 2.0 to 9.0% by mass of a flake copper powder with an average particle size of 20 to 150 μm and 1.5 to 3.7% by mass of a graphite powder with an average particle size of 40 to 80 μm; and then by performing sintering at a temperature of 950 to 1030° C., there will be formed in the bearing an iron alloy phase having ferrite at an area ratio of 20 to 85% and a remainder composed of pearlite (Patent document 2). However, since pearlite is a hard phase formed by the reaction of iron and graphite, a sliding partner shaft member cannot be sufficiently prevented from being damaged.